Many refineries face an abundant supply of lower alkanes, i.e., C1-C5 alkanes such as methane, and relatively few means of converting them to more valuable products. Moreover, vast reserves of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. Thus, there is great incentive to exploit these natural gas formations. However, most natural gas formations are situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage can make the use of these remote formations economically unattractive. In order to improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported than syngas.
The conversion of methane or natural gas to higher hydrocarbons is typically carried out in two steps. In the first step, methane or natural gas and an oxidant (water and/or molecular oxygen) are converted to a mixture of carbon monoxide and hydrogen (referred to as “synthesis gas” or “syngas”). In a second step, the synthesis gas is converted to higher hydrocarbons for example, by using the Fischer-Tropsch process or to methanol by using an alcohol synthesis process. Current industrial use of methane or natural gas as chemical feedstocks proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming, which is the most widespread process, or by dry (CO2) reforming or by autothermal reforming (with combination of O2 and steam). Other processes for making syngas include partial oxidation, catalytic partial oxidation, and advanced gas heated reforming.
Steam reforming is currently a major process used commercially for the conversion of methane to synthesis gas, proceeding according to Equation 1.CH4+H2OCO+3H2  (1)Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue. For many industrial applications such as Fischer-Tropsch process and alcohol synthesis process, the 3:1 ratio of H2:CO in the syngas product is problematic as being higher than the stoichiometric ratio necessary for the downstream conversion of the synthesis gas to fuels and/or to chemicals such as methanol, and the typically large steam reforming plants are not practical to set up at remote sites of natural gas formations.
The catalytic partial oxidation (CPOX) or direct partial oxidation of hydrocarbons (e.g., natural gas or methane) to synthesis gas has also been described in the literature. In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or molecular oxygen and introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Equation 2.CH4+½O2CO+2H2  (2)This H2:CO ratio is more useful than the ratio from steam reforming for the downstream conversion of the syngas to chemicals such as methanol and to fuels. The CPOX reaction is exothermic, resulting in high reactor operating temperatures well above 1000° C. Furthermore, oxidation reactions are typically much faster than reforming reactions. Methane residence times in steam reforming are on the order of 0.5-1 second, whereas for heterogeneously catalyzed partial oxidation, the residence time is on the order of a few milliseconds. For the same production capacity, syngas facilities for the partial oxidation of methane can be far smaller, and less expensive, than facilities based on steam reforming. This allows the use of much smaller reactors for catalytic partial oxidation processes than is possible in a conventional steam reforming process.
The current interest in partial oxidation processes, particularly employing a CPOX reactor, has resulted in various improvements in the technologies associated with syngas production, including catalyst composition, catalyst structure, reactor structure, and operating parameters. One aspect that has not received as much attention is the technology associated with the injection of feed gases into the partial oxidation reactor.
Because contact times in catalytic partial oxidation reactors are very short and can be on the order of milliseconds, an often desired component of a commercial scale CPOX operation is an apparatus to premix a hydrocarbon-containing feed, such as methane or natural gas, with a molecular oxygen-containing feed at high temperature, pressure and gas throughput in order to enable the catalytic reaction to proceed at short contact times so that the chemistry occurs at the correct stoichiometry throughout the catalytic zone. It is necessary to feed the reactant gases into the reactor under conditions of elevated temperature and pressure, and at the same time, it is desirable to mix the feed gases as completely as possible, so as to maximize the efficiency of the catalytic reaction. The same feed conditions that are conducive to efficient operation of the partial oxidation process, however, are conducive to reactions that are less desirable, and possibly even hazardous, such as the ignition and combustion of the feedstock.
In addition, gas-phase reactions in a CPOX reactor are particularly undesirable because they can increase the occurrence of undesired combustion reactions (producing steam and carbon dioxide), damage the catalyst, and accelerate its deactivation. In fact, these gas-phase reactions tend to generate an excessive gas peak temperature in the initial portion of the catalytic bed, resulting in decreased product selectivity. For this reason, it is particularly desirable to avoid gas-phase reaction during the pre-reaction stage (before contact to catalyst) and pre-ignition of the reactant gases. The pre-ignition point can be defined as the lower explosiveness concentration limits of the constituents in a mixture of a hydrocarbon and an oxidant. The lower explosiveness limits in the context of the production of synthesis gas are discussed in U.S. Pat. No. 4,620,940, which is hereby incorporated by reference herein for all purposes.
Moreover, the heated mixtures comprising molecular oxygen (very potent oxidant) and light hydrocarbons (combustible fluid), at pressures of interest for synthesis gas production, are highly reactive. In fact, maintaining the oxidant to combustible ratio within the desired limits is a critical element for the efficient operation of a CPOX process. Thus, it is often preferred to utilize mixing techniques or reactor designs that increase the controllability of the process as well as avoid pre-ignition and pre-reaction of the gases.
One technique, disclosed in U.S. Pat. No. 6,267,912, used in mixing the reactants is to place the mixing nozzles very close to the reaction zone such that there is a very short time between the reactants being mixed and contacting the catalyst. This technique often involves placing the mixing apparatus in close proximity to the reactor, which may make maintenance of the mixing apparatus difficult and requires that the mixer be designed to withstand the extreme environment of a partial oxidation reactor.
Thus, there remains a need in the art for methods and apparatus to improve the control and operation of a catalytic partial oxidation process, and more particularly to improve the means for delivering an oxidant stream and a combustible fluid stream to feed a catalytic partial oxidation reactor. Therefore, the embodiments of the present invention are directed to methods and apparatus that seek to overcome these and other limitations of the prior art.